Bonding apparatus and method

ABSTRACT

A bonding apparatus and method holds first and second bodies peripherally, one above the other, on respective shelves. A lower heat-transfer body is configured to lift the first body from below and press the first and second bodies against an upper heat-transfer body to enable bonding between the first and second bodies.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/495,114, entitled BONDING APPARATUS AND METHOD, filed by Steven M.Zuniga on Jun. 30, 2009, now U.S. Pat. No. 8,151,852 the contents ofwhich are hereby incorporated in their entirety. U.S. application Ser.No. 12/495,114 claims the benefit of U.S. Provisional Application Ser.No. 61/122,699, entitled APPARATUS AND METHOD FOR MANUFACTURING ANASSEMBLY HAVING A THIN LAMINA BONDED TO A BASE SUBSTRATE, filed byAditya Agarwal on Dec. 15, 2008, the contents of which are herebyincorporated in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to bonding systems, particular apparatus andmethods for forming a bonded structure incorporating a semiconductorlayer.

2. Background Information

A layered assembly incorporating a semiconductor wafer is a structureuseful for forming devices such as transistors, displays andmicroelectromechanical systems. A basic step of manufacturing thelayered assembly includes bonding a semiconductor wafer to a substrateby, for example, thermo compression, fusion, or anodic bonding.

In order to minimize the nucleation of multiple bonding fronts, andconcomitant concentration of voids in the bond, it is desirable tocontrol the development of contact between the opposing bonding faces ofthe wafer and substrate. Accordingly, in a conventional bondingapparatus, the semiconductor wafer and substrate are first arranged withtheir respective bonding faces separated across a narrow separation.Commonly the bonding apparatus provides small peripheral spacers thatare inserted between the stacked wafer and substrate to keep the bodiesseparate. After contact between the two facing surfaces has beeninitiated, the spacers are retracted.

The use of spacers may introduce difficulties into the bonding process.Mechanical contact between the spacers and the bonding surfaces of thetwo bodies may cause undesirable scratching, contamination, or particlegeneration and thereby compromise the quality of the bond and theperformance of the ultimate device. In more extreme cases, the spacersmay become stuck fast between and ruin the two bodies. The spacers maybe thin and prone to break, thereby reducing reliability of the bondingapparatus and compromising throughput due to downtime for maintenance.Also, managing the deployment and retraction of the spacers increasesthe complexity and expense of the bonding apparatus.

There is, accordingly, a need for a bonding system enabling managementof contact development between the two bodies being bonded withoutinterposed spacers.

SUMMARY OF THE INVENTION

A bonding system adapted to bond a first body to a second body comprisesa lower support shelf adapted to hold the first body and an uppersupport shelf adapted to hold the second body. The lower support shelfdefines a first interior space corresponding in lateral extent to thefirst body. The upper support shelf defines a second interior spacecorresponding in lateral extent to the second body. For some verticalplane passing through both of the first and second interior spaces, thesecond interior space has a lateral dimension, an intersection betweenthe vertical plane and the second interior space, that is longer than acorresponding lateral dimension, an intersection of the vertical planewith the first interior space. The support shelves are stationary andconfigured so that the first and second bodies disposed on the lower andupper shelves, respectively, are not in direct contact.

An upper heat-transfer body is disposed above and spaced apart from thesecond interior space. A lower heat-transfer body having an upperinterface is disposed under and spaced apart from the first interiorspace. The lower heat-transfer body is capable of rising so that theupper interface enters the first interior space. A lift assembly isconfigured to reversibly raise the lower heat-transfer body into thefirst interior space and press the lower heat-transfer body inopposition to the upper heat-transfer body.

In a bonding process joining the first and second bodies, the liftassembly raises the lower heat-transfer body to lift the first body. Thelower heat-transfer body presses the first body against the second bodyin contact with the upper heat-transfer body. As the first and secondbodies are sandwiched between the lower and upper heat-transfer bodies,force transmitted to the contact area between the first and secondbodies and heat transferred from the heat-transfer bodies to the firstand second bodies contribute to creating a bonded pair.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings,wherein identical reference symbols designate like structural elements,and in which:

FIG. 1 is a block diagram of a bonding system compatible with theinvention;

FIG. 2 is a sectional view of a bonding chamber compatible with theinvention;

FIG. 3 is a perspective view of the support plate and pins, without thefirst and second bodies, of the bonding chamber shown in FIG. 2;

FIG. 4 is a sectional view of the support plate and pins shown in FIG.3, taken through pins at each end of a diameter of the first or secondbody;

FIG. 5 is a sectional view of the support pin, shown in FIG. 3,supporting the first and second bodies;

FIG. 6 is a perspective view of an input or output shuttle compatiblewith a bonding system of the invention;

FIG. 7 is a flow diagram of an illustrative bonding process compatiblewith the invention;

FIG. 8 is a sectional view of the elevated lower susceptor lifting thefirst body in the bonding chamber shown in FIG. 2;

FIG. 9 is a sectional view of the elevated lower susceptor lifting thefirst body and the plunger tip depressing the second body in the bondingchamber shown in FIG. 2;

FIG. 10 is a sectional view of the elevated lower susceptor lifting thefirst body to expand the contact area with the second body in thebonding chamber shown in FIG. 2;

FIG. 11 is a sectional view of the bonded first and second bodiespressed between the lower and upper susceptors of the bonding chambershown in FIG. 2;

FIG. 12 is a sectional view of a semiconductor wafer implanted with ionsto create a cleave plane defining a lamina portion and a donor portion;

FIG. 13 is a sectional view of an ion-implanted semiconductor waferbonded to a receiver body in accordance with the invention; and

FIG. 14 is a sectional view of a bonded lamina-receiver assemblyfabricated in accordance with the invention.

Features in the figures are not, in general, drawn to scale.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

With reference to FIG. 1, in an illustrative embodiment, a bondingsystem 10 for bonding two substantially planar bodies comprises abonding chamber 20, accessible through an input port 35 and an outputport 45, an input shuttle 30 and an output shuttle 40, all anchored to abase plate 50.

A power apparatus 90 is operatively coupled to the input and outputports 35 and 45 and to components of the bonding chamber 20 that move,generate heat or assume a desired electrical potential. The componentsare described below with reference to FIG. 2.

An atmosphere apparatus 100 is configured to regulate the atmosphere inthe bonding chamber 20. Vacuum or pressure sensors (not shown) may bedisposed in the bonding chamber 20 and coupled to provide feedbackregulating operation of the atmosphere apparatus 100. The atmosphereapparatus 100 may, e.g., alternately admit components, such as inert gasor air, or remove components from the bonding chamber 20 to maintaindesired conditions. Practices for managing the temperature andcomposition of the environment in a container such as the bondingchamber 20 are known to those skilled in the art.

A computer system 80 is programmable and includes a main memory 82, acentral processing unit (CPU) 84, and a storage device 86, operativelycoupled to an input device 88 and a display 89. A graphical userinterface, software programs, and experimental parameters may be storedin the main memory 82. The computer system 80 is configured to cooperatewith the power apparatus 90, the atmosphere apparatus 100, components ofthe bonding chamber 20 described herein, and stress and temperaturesensors to generate process conditions for bonding in the chamber 20.The process conditions may encompass, e.g., temperatures ofheat-transfer bodies, bonding pressure between heat-transfer bodies,bonding atmosphere, bias voltage, heat-transfer body movement, plungermovement and pressure, which may be predetermined by a user and relayedthrough the is computer 80, for bonding performed in the chamber 20.

FIG. 2 shows an exemplary bonding chamber 20 with movable components inrespective rest positions. The chamber 20 is defined by a chamber floor110 which in use is affixed to the base plate 50 (FIG. 1), a chamber lid112, and a cylindrical outer wall 114 (shown in cross-section in thedrawing). The floor 110, lid 112 and outer wall 114 are illustrativelyof aluminum or stainless steel.

A lower heater 120 is supported by the chamber floor 110. A gimbal plate124 rests above the lower heater 120 and supports a lower susceptor 126.Support pins 130 are affixed to a support plate 132 circumscribing thelower susceptor 126. The support pins 130 are configured to bear a firstbody X1 beneath and without touching a second body X2. An upper heater140 and an upper susceptor 146 are borne by the chamber lid 112.Illustratively, electrically insulative material (not shown) separatesthe upper heater 140 and upper susceptor 146.

The lower and upper heaters 120 and 140 are coupled to the computersystem 80 (FIG. 1) and operable to generate electromagnetic radiation,thereby generating heat in and altering or maintaining respectivetemperatures of the lower and upper susceptors 126 and 146,respectively, in accordance with a predetermined process trajectory. Thelower and upper heaters 120 and 140 and susceptors 126 and 146 may befitted with temperature and stress sensors (not shown) to providefeedback to the computer system 80. The susceptors 126 and 146 areconfigured to function as heat-transfer bodies, transferring heat to thebodies X1 and X2 during a bonding operation in the bonding chamber 20.

The support pins 130 are illustratively of stainless steel or quartz,which may be preferable because of its dimensional stability over a widetemperature range. The susceptors 126 and 146 may be of a thermallyconductive material such as, e.g., graphite, silicon carbide,molybdenum, stainless steel, niobium, or aluminum. The heaters 120 and140 may be constructed from resistive heating elements, for exampleembedded in a thermally conductive block. Candidate materials andstructures for the heaters 120 and 140 and susceptors 126 and 146 areknown to those skilled in the art.

The power apparatus 90 may be capable of delivering, e.g., on the orderof 2,000 is watts or more to the heaters 120 and 140 to provide forrelatively rapid achievement of desired processing temperatures of thesusceptors 126 and 146. The power apparatus 90 may also be connectedwith the lower and upper susceptors 126 and 146 to allow application ofa bias voltage during bonding.

The chamber floor 110 is apertured to accommodate a susceptor liftassembly 138, coupled to the computer system 80 (FIG. 1), which isconfigured to engage a recess in the gimbal plate 124 and operable toreversibly lift the gimbal plate 124 and the lower susceptor 126. Thesusceptor lift assembly 138 has range sufficient to move the lowersusceptor 126 up to the lower surface of the first body X1 and continueupward until the second body X2 is held against the upper susceptor 146,sandwiched with the first body X1 between the two susceptors 126 and146. The susceptor lift assembly 138 is furthermore configured to pressthe lower susceptor 126 in opposition to the upper susceptor 146 toapply a bonding pressure at the interface between the first body X1 andthe second body X2, as predetermined by the user and communicatedthrough the computer system 80. The bonding pressure may be effected byapplying a stress of, for example, several hundreds or thousands ofPascals, for example 500, 1,000, 5,000 or 8,000 Pascals or greater. Thesusceptor lift assembly 138 is not limited by the type of apparatus usedto apply the bonding pressure and may incorporate, for example,electromechanical, pneumatic or hydraulic elements, known to thoseskilled in the art. As is known to those of skill in the art, a sealingmechanism (not shown) below the chamber floor 110, e.g., bellows, aroundthe susceptor lift assembly 138 may allow movement of the assembly 138through the floor 110 without compromising the atmosphere in the bondingchamber 20.

The gimbal plate 124 is configured to reorient the lower susceptor 126to conform to the lower surface of the first body X1 when the lowersusceptor 126 is pressing the first and second bodies X1 and X2 inopposition to the upper susceptor 146. In this way the gimbal plate 124affords passive compensation of the orientation of the lower susceptor126 for deviations from level of the upper susceptor 146 and imperfectplanarization of either or both of the bodies X1 and X2. The gimbalplate 124 may be configured so that its gimbal point coincides with thecenter of the top surface of the first is body X1 resting on the supportpins 130. In an alternative embodiment, the functions of the gimbalplate 124 and the lower susceptor 126 may be combined in a singleelement.

The chamber lid 112, upper heater 140 and susceptor 146 are centrallyapertured to accommodate a plunger 150 comprising a plunger tip 152engaged with a spring 154. The plunger tip 152 is illustratively ofsilicon carbide or stainless steel. A plunger drive 158, communicatingwith the computer system 80 (FIG. 1), is operatively coupled to thespring 154 to reversibly lower the plunger tip 152 downward to touch thesecond body X2. As is known to those of skill in the art, a sealingmechanism (not shown) above the chamber lid 112, e.g., bellows, aroundthe plunger 150 may allow movement of the plunger 150 through the lid112 without compromising the atmosphere in the bonding chamber 20.

The plunger drive 158 is configured to apply a specified force to, or toeffect a specified deformation in, the second body X2 in accordance withpredetermined parameters. The deformation may be, illustratively, up toabout 1 mm. The power apparatus 90 is optionally configured to apply abias, illustratively between 200 and 2,000 volts, between the lowersusceptor 126 and the plunger 150.

For clarity of illustration, FIG. 3 shows an illustrative arrangement ofthe support pins 130 adapted to bear two planar circular bodies. As usedherein, “planar body” and similar phrases refer to a body having athickness on the order of about 5% or less of a length characterizingits lateral extent.

In the embodiment, the support plate 132 has a generally circularaperture with four approximately semicircular outer cutouts 133 locatedaround the aperture at 90-degree intervals. The lower susceptor 126 isgenerally circular and rests inside the aperture in the support plate132. The lower susceptor 126 has four approximately semicircular innercutouts 134 co-located with the outer cutouts 133. Each inner cutout 134paired with its respective inner cutout 133 forms a circular nicheaccommodating a cylindrical support pin 130. The distal end, withrespect to the bodies X1 and X2, of each of four support pins 130 isaffixed to the support plate 132 in its respective outer cutout 133.

The proximal end of each of the support pins 130 is contoured, as shownin FIGS. 4 and 5, to support the first and second bodies X1 and X2. Eachof the pins 130 has a lower horizontal ledge 135 a and a lower verticalwall 135 b constituting a lower step 135. The lower vertical walls 135 bare cylindrically arcuate to accommodate a circular body. The lowersteps 135 are disposed around, and in aggregate constitute a lower shelf135 defining a first interior space 160. The first interior space 160corresponds in lateral extent, two dimensionally, to the first body X1.As used herein, an interior space's corresponding in lateral extent to abody means that the interior space is laterally larger than the body bya clearance desirable for easily loading and removing the body from theshelf defining the interior space. Illustratively, for thin circularbodies such as semiconductor wafers, such a clearance is present when aninterior space is about 0.05 to 1.0 mm greater in lateral extent thanits corresponding body.

Each of the pins 130 furthermore has an upper horizontal ledge 137 a anda cylindrically arcuate upper vertical wall 137 b constituting an upperstep 137. The upper steps 137 are disposed around, and in aggregateconstitute an upper shelf 137 defining, a second interior space 164. Thesecond interior space 164 corresponds in lateral extent, twodimensionally, to the second body X2.

The height of the upper vertical walls 137 b is sufficient to reliablycontain the second body X2 in place on the horizontal ledges 137 aagainst adventitious lateral force, or illustratively, equal to at leastabout half the thickness T2 of the second body X2.

The discrete support pins 130 leave most of the respective peripheriesof the first and second interior spaces 160 and 164 unencumbered. Thefree peripheries facing the input and output ports 35 and 45 allowsaccess to the interior spaces 160 and 164, between the support pins 130,by the input and output shuttles 30 and 40, as discussed below.

The second interior space 164 has a lateral dimension longer than acorresponding dimension in the first interior space 160, the lateraldimension and its corresponding dimension in the first interior space160 both lying in a common vertical plane. The larger extent of thesecond interior space 164 in the lateral dimension permits configurationof the lower susceptor 126 to pass through the first and second interiorspaces 160 and 164 and thereby lift both the first and second bodies X1and X2 disposed is on the lower and upper shelves 135 and 137.

In the embodiment, the second interior space 164 is larger in lateralarea than the first interior space 160 and overhangs it completely. Thelower and upper steps 135 and 137 delimit concentric circlesapproximately equal in size to the diameters of the first and secondbodies X1 and X2, respectively. The circle delimited by the lower steps135 may illustratively have a diameter approximately equal to that of astandard semiconductor wafer. The circle delimited by the upper steps137 may be larger in diameter than the circle delimited by the lowersteps 135 by, for example, approximately 5 mm or more.

In alternative embodiments, either or both of the lower and uppershelves 135 and 137 may be configured so that either or both of thefirst and second interior spaces 160 and 164 corresponds in lateralextent, two dimensionally, to a noncircular body, for example a squareor octagonal body. The lower susceptor 126 may be configured so that itsshape corresponds to the first body X1, and accordingly in embodimentsmay be, e.g., circular, square, or octagonal such as a corner-clippedsquare. The first and second interior spaces 160 and 164 may bedissimilar in shape. For example, the first body X1 may be a circle andthe second body X2 a square having an edge length equal to the diameterof the first body X1.

The vertical distance between the lower and upper horizontal ledges 135a and 137 a, which in the embodiment is the height of the lower verticalwall 135 b, is greater than the thickness T1 of the first body X1. Thisdistance creates an initial gap G between the bonding surfaces, whichare the portions of the upper surface U1 of the first body X1 and thelower surface L2 of the second body X2 that overlap when one of thebodies is projected onto the other. In the embodiment, the bondingsurfaces are the entire upper surface U1 of the first body X1 and itsvertical projection onto the lower surface L2 of the second body X2. Thegap G affords management of bond front nucleation and progression, forexample by evacuation of the bonding chamber 20 before contactinitiation, without intermediate spacers or other solid bodies touchingthe bonding surfaces on U1 and L2.

With reference again to FIG. 1, the input shuttle 30 is configured toconvey the is first and second bodies X1 and X2 to be bonded through theinput port 35 into the bonder chamber 20 and position them forprocessing on the lower and upper shelves 135 and 137. The provision ofopposing ports 35 and 45 and dedicated respective shuttles 30 and 40facilitates high throughput of the bonding chamber 20.

Turning to FIG. 6, the input shuttle 30 has a block 172 that slidestoward and away from the input port 35 along a horizontal drive assembly176. The horizontal drive assembly 176 is actuated by a horizontal drivemotor (not shown). An arm 184 is attached to the block 172 and is drivento move vertically by a vertical drive assembly 188 actuated by avertical drive motor (not shown). The arm 184 bears an end effector 190equipped with shuttle shelves 194 delimiting lower and upper footprintscorresponding to the first and second interior spaces 160 and 164,respectively. A sensor (not shown), such as an optical sensor, may beconfigured, e.g., on the arm 184 or the base plate 50 (FIG. 1) to sensethe presence or absence of bodies on the end effector 190 and/or thearrangement of bodies on the shuttle shelves 194.

The horizontal drive assembly 176 is mounted to the base plate 50(FIG. 1) in a position and orientation allowing the arm 184 to movethrough the input port 35 of the bonding chamber 20 and situate theshuttle shelves 194 in alignment over the second interior space 164. Thevertical drive assembly 188 is operable to lower the arm 184 so that theend effector 190 transfers bodies settled on the shuttle shelves 194onto the lower and upper shelves 135 and 137. In alternativeembodiments, the input shuttle 30 may be configured to effect horizontaland vertical motion of the end effector 190 by alternatives to thehorizontal and vertical drive assemblies 176 and 188 known to thoseskilled in the art.

The shuttle shelves 194 are located on the end effector 190 so as to fitbetween the support pins 130. For example, the embodiment of the shuttle30 depicted in FIG. 6 has shelves occupying an arc of less than 90degrees at the front and back ends of the end effector 190.

Referring again also to FIG. 1, the output shuttle 40 is structuredanalogously to the input shuttle 30. The horizontal drive assembly 176of the output shuttle 40 is mounted to the base plate 50 in a positionand orientation allowing the end effector 190 is of the output shuttle40 to be moved into position under the interior spaces 160 and 164 andlift bodies, as in a bonded pair created from the first and secondbodies X1 and X2, from the upper shelf 137 and move them out of thebonding chamber 20 through the output port 45. The complementary designof the support pins 130 and the input and output shuttles 30 and 40 andthe ability to load and unload from opposite sides of the bondingchamber 20, afforded by the distinct, dedicated input and outputshuttles 30 and 40, may enhance throughput of the bonding system 10.

FIG. 7 demonstrates steps in an exemplary process sequence for bondingtwo generally planar bodies X1 and X2 together. The computer system 80(FIG. 1) is operated to prepare the bonding system for use (step 205)by, e.g., positioning the input and output shuttles 30 and 40 outsidethe chamber 20, closing the input and output ports 35 and 45, puttingthe bonding chamber 20 into rest position by setting the lower susceptor126 below the first interior space 160 and retracting the plunger tipabove the second interior space 164 and out of the way of the inputshuttle 30. The computer system 80 may be furthermore operated toactivate the lower and upper heaters 120 and 140 to bring the respectivesusceptors 126 and 146 to a preliminary elevated temperature, forexample between 200° C. and 600° C., which is lower than thepredetermined susceptor bonding temperatures, and fill the chamber 20with a desired preliminary gaseous environment.

The first and second bodies X1 and X2 are prepared for bonding (step210). The bodies X1 and X2 each may have a high aspect ratio, on theorder of 100 or more, between a lateral dimension and thickness andopposing upper and lower surfaces that are substantially flat andparallel. According to the end use of the bonded pair, preparation of abody may comprise procedures known to those skilled in the art, e.g.,growing or casting the body to a custom or standard diameter, e.g., 150mm, 200 mm or 300 mm, 400 mm or greater, compatible with one of thelower and upper shelves 135 and 137; removing the body from a largeringot such as by slicing; etching one or more faces of the bodies X1 andX2 to a desired surface roughness; diffusion doping the body to createan n-type or p-type layer; fabricating wiring; depositing a transparentconductive oxide or an amorphous silicon layer; depositing or growing anoxide or nitride layer; depositing a is conductive layer or stack oflayers; and cleaning surfaces of the body such as by megasonic rinsingwith spin drying or otherwise treating surfaces to remove chemicalresidues and particles, for example any particles exceeding 2 μm indiameter.

Depending on the type of bonding to be accomplished in the bondingchamber 20, one or both of the bodies X1 and X2 may be coated on oneside with an adhesive or a fusible substance that melts during bondingto join the two bodies; or one or more surfaces of the bodies mayactivated with plasma. Body preparation for techniques such as thermocompression, adhesive, plasma and anodic bonding are described inco-owned U.S. patent application Ser. No. 12/335,479, Agarwal et al.,“Methods of Transferring a Lamina to a Receiver Element,” the disclosureof which is incorporated herein in its entirety by reference. As usedherein, the upper surface U1 of the first body X1 and the lower surfaceL2 of the second body X2 refer to the uppermost or lowermost,respectively, surface of the respective body when it is placed on thelower or upper horizontal ledges 135 a and 137 a, respectively, whetherthe surface U1 or L2 represents the bulk, interior material of therespective body, X1 or X2, or a surface coating or layer, for example,an applied material.

One or both of the bodies X1 and X2 may be of an electronics-gradesemiconductor material, such as silicon, germanium, silicon germanium,or a III-V or II-VI compound such as gallium arsenide or indiumphosphide. The semiconductor material may have a monocrystalline,polycrystalline, multicrystalline or microcrystalline microstructure.Polycrystalline and multicrystalline semiconductors are understood to becompletely or substantially crystalline. A polycrystalline semiconductorbody is comprised of crystals on the order of 1 mm in size. Amulticrystalline semiconductor body has a grain size on the order of1,000 Angstrom units. By contrast, a microcrystalline semiconductor maybe fully crystalline or may include fine microcrystals in an amorphousmatrix. Microcrystals in a microcrystalline semiconductor body are onthe order of 100 Angstrom units in size. One of the bodies X1 and X2 maybe of glass, ceramic, metal, metal-containing compound, plastic,metallurgical silicon, or a layered stack of diverse materials.

With reference again to FIG. 6, the prepared first and second bodies X1and X2 is are placed flat onto the shuttle shelves 194 of the inputshuttle end effector 190 in a substantially parallel, spaced-apartstack. (step 215) The bodies may be placed onto the shuttle shelves 194manually or by a robot, for example. A sensor may be activated to verifycorrect positioning of the first and second bodies X1 and X2 on the endeffector 190.

Once correctly positioned on the end effector 190, the bodies X1 and X2may be preheated. The input shuttle 30 may be operated to position thefirst and second bodies X1 and X2 in a preheat station (not shown in thedrawings). The preheat temperature, and respective temperatures of otherenvironments in which the bodies reside during the process sequence, arechosen for compatibility with process steps in light of considerationssuch as time efficiency and thermal budget. For example, a higherpreheat temperature may shorten the overall bonding process time butalso prematurely initiate other thermally activated processes morepreferably occurring in later process steps.

The bonding chamber 20 is prepared for loading (step 220) by increasingthe output of the heaters 120 and 140 to bring the susceptors 126 and146 to respective bonding process temperatures, either of which may bee.g., on the order of 200° C., 300° C., 400° C., 500° C., 600° C., 700°C., 800° C., or greater. Illustratively the bonding temperature of bothof the susceptors 126 and 146 are approximately equal and lie between200° C. and 800° C., between 350 and 550° C., or between 400 and 500° C.The bonding chamber 20 is purged with nitrogen as the input port 35 isopened.

The input shuttle 30 is operated to load the bodies X1 and X2 onto thesupport pins 130 (FIG. 2) in the bonding chamber 20. (step 225) Thecomputer 80 (FIG. 1) directs the vertical drive assembly 188 (FIG. 6) toadjust the elevation of the end effector 190 for positioning over thesupport pins 130. The horizontal drive assembly 176 (FIG. 6) is directedto advance the arm 184 through the input port 35 (FIG. 1) until the endeffector 190 is positioned for loading. The vertical drive assembly 188then lowers the arm 184 until the bodies X1 and X2 are settled on thelower and upper shelves 135 and 137 (FIG. 5), respectively. A sensor(not shown) on the input shuttle 30 or inside the bonding chamber 20 mayverify proper placement of the bodies. When placement is satisfactory,the horizontal drive assembly 176 retracts the end effector 190 from theis bonding chamber 20.

Next, the input port 35 is closed, the nitrogen purge ended and thebonding chamber 20 evacuated. After sufficient time for thermalequilibration in the bonding chamber has passed, the susceptor liftassembly 138 is activated to raise the lower susceptor 126 until thelower susceptor 126 has lifted the first body X1 from the lower shelf135. (step 230) With reference to FIG. 8, at the end of step 230 thelower susceptor 126 has come to rest holding the upper surface U1 of thefirst body X1 at a predetermined separation Q from the lower surface L2of the second body X2, which remains on the upper shelf 137.

Referring back to FIG. 2, the plunger drive 158 is next activated todrive the plunger 150 downward so that the tip 152 touches the uppersurface U2 of the second body X2 with force sufficient to bow the secondbody X2, forming a convexity C approximately at the center of its lowersurface L2, facing the first body X1. (step 235) With reference to FIG.9, at the end of step 235, the separation Q remains between the uppersurface U1 and the lower surface L2 near the horizontal ledges 135 a.Under the plunger tip 152 the first and second bodies X1 and X2 approachcloser than the predetermined separation Q.

The susceptor lift assembly 138 resumes upward motion to bring thesecond body X2 against the upper susceptor 146 (FIG. 2) (step 240). Inone approach, the elevation of the plunger drive assembly 158 is fixedduring step 240. When the first body X1 rises sufficiently to close theseparation Q under the convexity C, contact is initiated between theupper surface U1 and the lower surface L2. Ideally, the contact occursover a continuous circular interface region R, as shown in FIG. 10. Asthe first body X1 lifts the second body X2 off the upper shelf 137 andcontinues upward, the spring 154 is compressed and the force of the tip152 against the upper surface U2 of the second body X2 increases. Thecontact front around the circular interface region R may advanceapproximately radially. FIG. 10 shows the second body X2 supported bythe first body X1 through the contact region R. The susceptor liftassembly 138 continues upward until the contact region R substantiallycovers the entire upper surface U1 and the upper is surface U2 of thesecond body X2 is against the upper susceptor 146, as shown in FIG. 11.

Alternatively, during step 240 the plunger drive 158 (FIG. 2) isoperated to track the position of the upper surface U2, thus retractingas the second body X2 rises, thereby maintaining a constant forcebetween the tip 152 and the upper surface U2. Also, the contact region Rmay be precipitated by the initial descent of the plunger tip 152 instep 235 instead of in step 240.

In step 245, the first and second bodies X1 and X2 are held underpressure between the respective lower and upper susceptors 126 and 146(FIG. 2) until the contact area R is converted to a bond. Thecompression may be maintained, for example for a predetermined period onthe order of one minute, five minutes, thirty minutes or more. Ingeneral, completion of the bond at the upper surface U1 of the firstbody X1 and the lower surface L2 of the second body X2 involves theapplication of pressure between the susceptors 126 and 146 and thetransfer of heat from the susceptors 126 and 146 through the respectivebodies X1 and X2 to the contact region R. Step 245 may additionallyinvolve, for example, the application of a bias voltage across thebodies X1 and X2 to achieve anodic bonding. Alternatively, the bond iscompleted by the fusion and solidification of material at the uppersurface U1 or lower surface L2, for example, a preapplied metal coatingapplied to one or both of the surfaces U1 and L2 during step 210.Diverse bonding techniques are described in U.S. patent application Ser.No. 12/335,479, earlier incorporated by reference.

When the bond is complete, the susceptor lift assembly 138 (FIG. 2) isretracted to remove the upper surface U2 of the second body X2 from theupper susceptor 146. The plunger drive 158 may be engaged to push theplunger assembly 150 downward to help separate the upper surface U2 ofthe second body from the upper susceptor 146. The plunger tip 152 mayfurthermore follow the upper surface U2, either by passive extension ofthe spring 154 or under force from the plunger drive 158. In this case,the presence of the plunger tip 152 on the upper surface U2 may inhibitundesired lateral motion on the lower susceptor 126 by the bonded pairformed from X1 and X2.

When the unbonded portion of the lower surface L2 reaches the horizontalledge 137 a of the upper shelf 137 (FIG. 5), the bonded pair comes torest and the lower susceptor 126 continues to its rest position belowthe lower shelf 135. (step 250) At this point, the plunger tip 152 maybe retracted into the upper susceptor 146.

Referring again to FIG. 1, the power apparatus 90 and the atmosphereapparatus 100 of the bonding system 10 are operated to bring theenvironment in the bonding chamber 20 to a suitable temperature andcomposition for opening the chamber 20, for example by cutting offvacuum or backfilling with an inert gas such as nitrogen. The outputport 45 is opened and the output shuttle 40 is operated to lift thebonded pair off the upper shelf 137 (FIG. 4) and remove it from thechamber 20. (step 255)

In an exemplary embodiment adapted to make a photovoltaic-readysilicon-glass structure by anodic bonding in the bonding system 10, thelower step 135 is configured to hold a wafer X1 about 150 mm in diameterwith a clearance of about 0.2 mm around the perimeter. The lowerhorizontal wall 135 a is about 2 mm in radial extent and the lowervertical wall 135 b is greater than about 3 mm tall. The upper shelf 137is adapted to contain a second body X2 approximately 1.0 mm to 3.5 mm inthickness and 154 mm in diameter, also with a clearance of about 0.2 mmaround the perimeter. The upper horizontal shelf 137 a is about 2 mm inradial extent and the upper vertical wall 137 b is about 2 mm tall. Thelower and upper steps 135 and 137 are shaped in the proximal ends ofrespective quartz support pins 130.

The power apparatus 90 incorporates 60 mA power supply configured toapply a bias voltage between the lower and upper susceptors 126 and 146.The plunger tip is 10 mm in diameter and of silicon carbide. Thesusceptors 126 and 146 are also of silicon carbide, which resists attackby sodium ions in contact with a glass body under bias.

With reference again to FIG. 7, in step 205, the lower 126 and uppersusceptors 146 in the bonding chamber 20 are heated to a preliminaryelevated temperature of about 450° C. The bonding chamber 20 is filledwith nitrogen gas at approximately atmospheric pressure.

In step 210, the first body X1 is prepared by providing a round siliconmonocrystal, referred to as a silicon wafer, illustratively on the orderof 0.7 mm thick and 150 mm in diameter. The silicon wafer X1 isfurthermore implanted with 8×10¹⁶ is hydrogen ions/cm² through whatbecomes the upper surface U1 when the silicon wafer X1 is placed in thebonding chamber 20. With reference to FIG. 12, the implanted hydrogenions define a cleave plane P, illustratively about 3.0 μm below theupper surface U1 and defining a lamina portion A of the silicon wafer,between the cleave plane P and the upper surface U1 and a donor portionD between the cleave plane P and the lower surface L1 of the siliconwafer X1. The lamina portion A is subject to exfoliation from the donorportion D at the cleave plane P, e.g., upon annealing at hightemperature.

In alternative embodiments, the silicon wafer may be implanted with,e.g., helium ions, alone or in addition to hydrogen, and the cleaveplane may be from about 0.2 μm to 20 μm, or between 1 μm and 5 μm, belowthe upper surface U1 of the silicon wafer. The total implanted ionconcentration may alternatively be between about 4×10¹⁶ and 2×10¹⁷ions/cm. Details of creating lamina portions in semiconductor materialsby ion implantation and subsequent exfoliation are described in co-ownedU.S. patent application Ser. No. 12/407,064, Petti et al., “Method toMake Electrical Contact to a Bonded Face of a Photovoltaic Cell,” thedisclosure of which in its entirety is incorporated herein by reference.

The second body X2, referred to here as a receiver element, is a roundsubstrate of borosilicate glass, illustratively about 1.1 mm thick and400 mm in diameter. The close match between the thermal expansionproperties of borosilicate glass and the wafer material facilitatepost-bonding handling of the bonded pair. Alternatively, the receiverelement could be of soda lime glass.

The upper surface U1 of the semiconductor wafer X1 (as shown in FIG. 12)or the lower surface L2 of the receiver element X2 is illustrativelycovered, e.g., by sputtering, with a conductive and/or reflectivemetallic material to form a layer M. In an alternative approach,material added to both of the upper surface U1 and the lower surface L2constitutes the layer M. The material in the layer M may be of titaniumor aluminum or silicides thereof. Alternatively or additionally, themetallic layer may include a metal such as chromium, molybdenum,tantalum, zirconium, vanadium, tungsten, nickel, copper, silver,ruthenium, niobium, cobalt, zinc, indium, antimony, tin, lead, iron oran alloy, oxide, silicide or other compound thereof, or combinationthereof. The layer M is may be between about 30 Angstrom units and 2,000Angstrom units thick, for example about 100 to 200 Angstrom units thick.Candidate materials for the semiconductor wafer X1, the receiver elementX2, the layer M and its disposition are discussed in are described inU.S. patent application Ser. No. 12/335,479, earlier incorporated byreference, and co-owned U.S. patent application Ser. No. 12/057,265,Herner, “Method to Form a Photovoltaic Cell Comprising a Thin LaminaBonded to a Discrete Receiver Element,” the disclosure of which in itsentirety is incorporated herein by reference.

In steps 215 through 225, the wafer X1 and receiver element X2 aredisposed on the input shuttle 30, the setpoint for both of the lower andupper susceptors 126 and 146 is raised to a predetermined bondingprocess temperature of 450° C., and the wafer X1 and receiver element X2are transferred to the support pins 130 by the input shuttle 30. Thebonding chamber 20 is brought to a vacuum of about 10⁻⁴ millibar. Oncethe susceptors 126 and 146 have reached the process temperature, in step230 the lower susceptor 126 is raised to lift the wafer X1 off the lowerhorizontal ledge 135 a until the upper surface U1 and the lower surfaceL2 of the receiver element X2 are separated by the predeterminedseparation Q. After about 30 seconds of contact between the lowersusceptor 126 and the lower surface L1 of the wafer X1, the plungerdrive 158 lowers the tip 152 to produce the convexity C, reaching about0.05 mm downward, in the lower surface L2 of the receiver element X2 instep 235. The convexity C does not span the separation Q.

In step 240, the lower susceptor 126 is raised further to lift the waferX1, which contacts the convexity C and further lifts the receiverelement X2 until the wafer and receiver element are held togetheragainst the upper susceptor 146. In step 245, as the wafer X1 andreceiver element X2 reach thermal equilibrium with the susceptors 126and 146, the susceptor lift assembly 138 creates a bonding stress equalto about 5,000 Pa between the lower and upper susceptors 126 and 146.The power apparatus 90 is activated to apply a bias voltage of about 500V between the lower and upper susceptors 126 and 146 for a predeterminedbiasing interval of, e.g., about 5 minutes.

In an alternative embodiment, instead of predetermining a biasinginterval, the is computer system 80 may be configured to control thebias voltage in response to a monitored current passing across thecontact area R. As bonding progresses, an oxide layer may form at thecontact R, causing the current to decrease after attaining a peak valueof about 10 to 30 mA and affording a metric by which to evaluate bondingprogress.

When bonding is complete the first and second bodies have become abonded wafer-receiver pair X12. After shutting off the bias voltage, thebonded wafer-receiver pair X12 is settled on the upper shelf 137 as thelower susceptor 126 retreats toward the support plate 132. (step 250)Finally, the bonded pair X12 is removed from the chamber 20 as describedearlier for step 255.

With reference to FIG. 13, the bonded wafer-receiver pair X12 issuitable for further treatment to form a bonded lamina-receiverassembly. Subjecting the bonded pair X12 to proper exfoliatingconditions causes the lamina portion A to separate from the donorportion D, thereby rendering, in the embodiment, a bondedlamina-receiver assembly Y comprising the receiver element X2 with a3-μm-thick lamina portion A affixed thereto, as shown in FIG. 14.Additional processing that may be performed in order to completefabrication of the photovoltaic device are described in U.S. applicationSer. Nos. 12/335,479 and 12/057,265, earlier incorporated by reference.

The separated donor portion D may be implanted again to define aconsecutive cleave plane, through its lower surface L1 or new uppersurface U1′ exposed by exfoliation of the lamina A, and used as a firstbody in a subsequent bonding process. Illustratively, the semiconductorwafer X1 may be implanted, bonded to a receiver, and exfoliated multipletimes. The cooperation of the moveable lower susceptor 126 with theconfiguration of the lower shelf 135 gives the bonding chamber 20flexibility with regard to wafer thickness T1 that makes it suitable forbonding wafers to receivers in between repeated exfoliations. When anexfoliation renders the separated donor portion D thinner than somecritical value, illustratively 300 μm, easily handled in the bondingchamber 20, the wafer X1 may be sold for repurposing or otherwisedisposed of.

The bonding system 10 and related method afford a bonding method withoutthe surface damage hazards, bonder chamber complexity, and reducedthroughput associated with interposing spacers between the semiconductorand receiver element.

Although specific features of the invention are included in someembodiments and not in others, it should be noted that individualfeature may be combinable with any or all of the other features inaccordance with the invention. Furthermore, other embodiments arecompatible with the described features. For example, the upper susceptor146 may be configured to descend toward the upper surface U2 of thesecond body X2.

It will therefore be seen that the foregoing represents a highlyadvantageous approach to bonding, particularly for bonding planar bodiessuch as wafers and substrates. The terms and expressions employed hereinare used as terms of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof, butit is recognized that various modifications are possible within thescope of the invention claimed.

What is claimed is:
 1. A method of treating a first body having an uppersurface, a lower surface, and a thickness and a second body having anupper surface and a lower surface, the method comprising: configuring alower shelf, having a vertical wall and a horizontal ledge, around afirst interior space corresponding in lateral extent to the first body;configuring an upper shelf, having a vertical wall and a horizontalledge above the lower shelf and around a second interior space,corresponding in lateral extent to the second body, the second bodybeing larger in lateral extent than the first body, so that therespective horizontal ledges of the upper and lower shelves areseparated vertically by a distance greater than the thickness of thefirst body; disposing a lower heat-transfer body, having an upperinterface, under and spaced apart from the first interior space;disposing an upper heat-transfer body, having a lower interface aboveand spaced apart from the second interior space; disposing the lowersurface of the first body on the horizontal ledge of the lower shelf;disposing the lower surface of the second body on the horizontal ledgeof the upper shelf; and raising the lower heat-transfer body to lift thefirst body above the horizontal ledge of the lower shelf, the first bodyin turn lifting the second body from the upper shelf, and press thefirst and second bodies together against the upper heat-transfer bodyuntil the first and second bodies form a bonded pair.
 2. The method ofclaim 1 wherein the upper heat-transfer body has an aperture at a rightangle to the lower interface and further comprising: configuring aplunger to reversibly descend through the aperture toward the secondinterior space; and lowering the plunger to contact the second body,thereby forming a convexity in the lower surface of the second body,contact between the first body and the second body initiating at theconvexity and spreading over substantially the entire upper surface ofthe first body as the second body approaches the upper heat-transferbody.
 3. The method of claim 1 further comprising imposing a biasvoltage across the first and second bodies pressed together between theupper and lower heat-transfer bodies to effect an anodic bond.
 4. Themethod of claim 1 wherein one of the first and second bodies is asemiconductor wafer.
 5. The method of claim 1 wherein at least one ofthe upper surface of the first body and the lower surface of the secondbody represents an applied material distinct in composition fromrespective interior compositions of the first and second bodies.
 6. Themethod of claim 1 wherein the first body is a semiconductor wafer andcontains hydrogen or helium implanted beneath the upper surface,defining a cleave plane.
 7. The method of claim 6 wherein the secondbody is a receiver and further comprising annealing the bonded pair toeffect exfoliation of a lamina from the first body, thereby rendering abonded lamina-receiver assembly.
 8. The method of claim 6 wherein thefirst body is of monocrystalline silicon.
 9. The method of claim 6wherein the defined cleave plane is at least 1 micron below the uppersurface of the wafer, the receiver is of glass and the bondedlamina-receiver assembly is suitable for fabricating a photovoltaicdevice.
 10. The method of claim 1 further comprising couplingelectromagnetic energy to the lower and upper heat transfer bodies.